I've been very impressed with the 5X's altimeter. Today is a great example of why.
I have auto-calibrate turned off. I calibrated it manually yesterday, at home. Today a friend and I drove two hours through a storm to hike; the trailhead was at approximately 1,300 feet and we made it to the 4,000 level where we ultimately lost the trail under deep snow with more falling out of the sky on us. We turned back, the weather improved when we left the mountains, and were in the sun on the drive back. My home is 60 feet above MSL; when I got home the watch reported 52 feet.
With the wildly changing weather and 2,800 feet of ascent and descent, to be wrong by only 8 feet is amazing.